As dimensions shrink and circuit densities increase in circuit board and card manufacturing, the problems heretofore associated with and limited to IC manufacture are now increasing relevant to circuit board and card manufacture. For example, Class-100K and 10K cleanrooms are typically used in card and board manufacturing. Circuit cards and boards are 10" by 15", and even 24" by 28", and larger, with thickness of 1-5 mils, 2 mil feature sizes, and circuit densities that result in ten or more linear feet of Cu wiring per square inch of card or board surface.
In this 2 mil and smaller geometry environment, a dust particle that would have gone unnoticed in a 4 mil geometry environment can result in incomplete etching during subtractive circuitization resulting in an electrical open.
A small volume of still, particle free air, with no internal (local) source of particles is the cleanest possible environment for high circuit density circuit card and board manufacturing.